Skin has two major layers: the epidermis and the dermis. The epidermis is the outer most layer of skin. The dermis is the lower layer of skin that contains collagen and elastin fibers that provide strength to skin and where skin vasculature and nerves are found. The epidermis has five layers. The outermost layer is the stratum corneum, followed by the stratum lucidum, stratum granulosum, stratum spinosum, and, finally the stratum basale as the deepest layer next to the dermis. The stratum basale contains cells that continuously divide and form new keratinocytes to replace those that are being shed. The stratum basale also contains melanocytes that produce skin color. The stratum spinosum contains the keratin producing cells that were formed in the stratum basale. The stratum granulosum is where keratin and other biological materials are produced that help to waterproof skin. The stratum lucidum is found in thicker skin and is formed of flattened dead cells. It reduces friction between stratum corneum and stratum granulosum. 
The stratum corneum is largely responsible for skin barrier function. It was once believed that the stratum corneum was biologically inert. However, it is now recognized that it has an intricate chemical and physical biology despite the fact that the corneocytes (keratinocytes that have become cornified) which make it up are dead cells. Maintaining a healthy stratum corneum is vital to achieving healthy skin and its associated attractive appearance.
The structure of the stratum corneum has often been analogized to a “brick and mortar” type of construction with corneocytes forming the bricks. About 12 to 16 layers of corneocytes form a protein complex with an organized matrix comprised of threads of keratin that can retain considerable amounts of water between the threads. In general, each corneocyte has a diameter of about 1 micron which may vary depending on the individual's age, exposure to environmental conditions, or other factors. Keratinocytes proliferate in the stratum basale and migrate through the layers of the epidermis to the skin surface and replace keratinocytes that become cornified. While the keratinocytes migrate through the stratum spinosum and stratum granulosum, lamellar bodies are formed within. When they mature to the stratum corneum, enzymes degrade the outer envelope of the lamellar bodies to release free fatty acids and ceramides to fuse together in the stratum corneum to form a cornified envelope containing a continuous layer of lipids. Because there are two types of lipids, this layer is referred to as a lamellar lipid bilayer. This bilayer plays a major role in maintaining the barrier properties of skin and is often referred to as the mortar component in the brick and mortar analogy. Corneocytes are surrounded by a cell envelope that is composed primarily of proteins loricim and involucrin that contain extensive linkages that create an insoluble barrier. Attached to the cell envelope is a layer of ceramide lipids that repel water. Because the lamellar lipid bilayer also repels water, water molecules are held between the cell envelope lipids and the lipid bilayer. This helps maintain the water balance in the stratum corneum by trapping water molecules instead of letting them absorb into the lower layers of the epidermis. These proteins contain extensive links between each other making the cell envelope the most insoluble structure of the corneocyte. The “rivets” that hold the corneocytes together are specialized protein structures called corneodesmosomes, which are the major structures that must be degraded for the skin to shed in a process called desquamation. Natural moisturizing factor (NMF) is a collection of water-soluble compounds that are only found in the stratum corneum. These compounds comprise approximately 20 to 30 percent of the dry weight of the corneocyte. NMF components absorb water from the atmosphere and combine it with their own water content allowing the outermost layers of the stratum corneum to stay hydrated despite exposure to the elements. Because NMF components are water-soluble, they are easily leached from the cells with water contact, which is why repeated contact with water actually makes the skin drier. The lipid layer surrounding the corneocyte helps seal the corneocyte to prevent loss of NMF.
The desquamation or exfoliation process of the stratum corneum is actually very complex and only parts of this process are fully understood. It is known that several enzymes degrade the corneodesmosomes in a specific pattern. While water and pH are known to play a significant role in the activation of enzymes necessary to start the exfoliation process, the exact nature of the enzymes and the activation necessary to start the exfoliation process still remains unknown.
Accordingly, there is great interest in formulating products for topical application to skin that will correct, supplement, and maintain skin barrier function, minimize loss of NMF, and supplement the skin's natural biological process of keratinocyte generation and cornification in order to optimize the appearance of heathy skin.
It has been discovered that the stratum corneum and underlying layers of the epidermis can be significantly strengthened and thickened by formulating topical products that contain certain ingredients that interact with each other to form a micro-mesh-like structure in the form of three dimensional structures of interlocked spheres that are associated with each other to form a network.